The present invention relates to a novel block copolymer, a rubber composition containing the block copolymer, and methods of producing the same. The block copolymer of the present invention can compatibilize mutually incompatible diene rubbers and can further provide a rubber composition having improved tensile strength, abrasion resistance, etc., without decreasing the vulcanization activity and further without changing the inherent properties of the polymer blend, for example, the low fuel consumption, wet braking performance, etc., by blending the block copolymer to a diene rubber. The block copolymer and rubber composition of the present invention may be suitably used in the fields of tire rubber etc.
The present invention further relates to a pneumatic tire improved in abrasion resistance and chipping resistance.
In the past, rubber for use for tires of automobiles etc. has been required to have various performances such as strength, abrasion resistance, low heat buildup, high impact resilience, wet skid resistance, and chipping resistance. However it is difficult to satisfy these required performances by a single type of rubber, and therefore, for example, attempts have been made to balance and improve the performances by using blends of several diene rubbers such as blends of natural rubber and styrene-butadiene copolymer rubber (SBR). However, even if in the case of diene rubbers, since the rubbers are of different types, they are not necessarily fully compatible and sometimes are incompatible. In particular, when diene rubbers are incompatible, even if they are homogeneously mixed, it is difficult to obtain sufficient vulcanized properties.
Therefore, in recent years, various methods have been proposed for compatibilize incompatible diene based rubbers. For example, Japanese Unexamined Patent Publication (Kokai) No. 7-188510 discloses a method of using, as a type of compatibilizing agent, a block copolymer composed of styrene-butadiene copolymer (SBR) block having a bound styrene content of not more than 30% by weight and an amount of vinyl bonds of the butadiene part of not more than 40 mol % and an SBR block with an amount of bonded styrene of not more than 30% by weight and a bound vinyl content of the butadiene part of not less than 70 mol %. However, in this method, there is the disadvantage that, while it is possible to compatibilize natural rubber and SBR, the vulcanization rate becomes slower and further the improvement in the abrasion resistance, tensile strength, and other vulcanized properties is not sufficient.
Japanese Unexamined Patent Publication (Kokai) No. 64-81811 reports a block copolymer composed of a polyisoprene block having a bound 1,4-cis content of not less than 65 mol % and an SBR block having a bound styrene content of 5 to 40% by weight and a bound vinyl content of the butadiene part of not less than 55 mol %. This block copolymer, when used as a compatibilizing agent for natural rubber and SBR, cannot give a sufficient effect of improvement of the abrasion resistance, tensile strength, etc. According to studies by the present inventors, this block copolymer only shows a single transition point in analysis by differential scanning calorimetry (DSC). This is deduced to be the cause for why a sufficient compatibilizing action cannot be obtained between different types of rubbers.
In recent years, improvement of various performances has been sought from rubber compositions for tires of automobiles and the like. Therefore, the practice has been to blend several types of polymers in rubbers for tire blends etc. When these polymers are incompatible, a phase separation is present at the interface. In most cases, it is believed that this interface becomes a starting point of breakage and has an adverse effect on the tensile strength, tear strength, abrasion resistance, etc. However, in rubber products such as tires, the unique processing, i.e., vulcanization is involved, and therefore, it is not possible to apply as is the molecular design of block copolymers for controlling a phase structure such as is done in ordinary rubber/resin and resin/resin blends. However, the problem of the phase separation at the interface of rubber/rubber blends, has not been sufficiently studied and no method for solving the problem has been discovered either.
In the past, the decrease in the breaking strength due to the incompatibility of a polymer blend obtained by blending in a block copolymer has not been sufficiently studied. Formulating a small amount of a block copolymer of polybutadiene (BR) and polyisoprene (IR) in a blend of natural rubber (NR)/polybutadiene rubber (BR) has only been briefly described in J. Apply. Polym. Sci., 49 (1993) and RCT. 66 (1993). The compositions of the block copolymers used in these references are not satisfactory in performance in actual use due to the insufficient compatibility with BR. Further, experiments have been made on adding cis-BR to incompatible polymer blends of cis-BR/SBR to improve the abrasion resistance, but there is a limit to the amount of the cis-BR added due to the decrease in the wet braking performance, and therefore, there have been problems in actual use.
In consideration of the above situation, the present inventors previously proposed tire trend compositions containing an A-B type block copolymer (see Japanese Unexamined Patent Publication (Kokai) No. 7-188510 and Japanese Unexamined Patent Publication (Kokai) No. 8-134267).
On the other hand, in the past, blends of several types of polymers have been used for treads of pneumatic tires to improve various aspects of their performance. In particular, when trying to improve the abrasion resistance and heat buildup property, a blend of styrene-butadiene copolymer rubber (SBR) or polybutadiene rubber (BR) having a low glass transition temperature (low Tg) is often used with a natural rubber (NR) or polyisoprene rubber (IR). However, in such a blend, there is the disadvantage that, while the abrasion resistance and heat buildup property are improved, the chipping resistance is inferior due to the low breaking strength and breaking energy.
As mentioned above, the breaking strength and breaking energy are low in a blend of polymers due to the failure of the polymers to mix well with each other (incompatible). Since they are incompatible, there is a phase separation at the interface in the blend. This interface serves as the starting point for breakage and invites a decrease in the breaking energy. Further, since they are incompatible, the so-called xe2x80x9cislands in the seaxe2x80x9d state is present in the blend, and therefore, the carbon black introduced into the blend for reinforcement purposes are localized in different areas and the problem arises that the breaking strength is decreased. Therefore, as a measure against this, it has been proposed to add carbon black having a small particle size and low structure into the blend, but in this case as well it has not been possible to secure a sufficient abrasion resistance.
Accordingly, the object of the present invention is to provide a novel polymer compatibilizing agent which can eliminate the above problems in the prior art, which can compatibilize diene based rubbers having insufficient compatibility, and which can improve the abrasion resistance, tensile strength, etc. without slowing the vulcanization rate.
Another object of the present invention is to provide a rubber composition having an excellent vulcanization rate and superior abrasion resistance or tensile strength.
Another object of the present invention is to provide a rubber composition which, when used for a tire, can improve the abrasion resistance and chipping resistance without adversely affecting the low fuel consumption, wet braking performance, and other properties.
Still another object of the present invention is to provide a pneumatic tire which is improved in abrasion resistance and chipping resistance by adding a specific block polymer to a blend of incompatible polymers to form a tread rubber having a fine phase structure.
In accordance with the present invention, there is provided a block copolymer (1) containing a polymer block A of a conjugated diene and a random copolymer block B of a conjugated diene and an aromatic vinyl compound, (2) having a weight ratio (A:B) of the polymer block A and the copolymer block B of 5:95 to 95:5, (3) having a bound aromatic vinyl compound content in the copolymer block B of 1 to 50% by weight, (4) having a weight average molecular weight (Mw) of 100,000 to 5,000,000, and (5) having at least two transition points measured by differential scanning calorimetry in the range of xe2x88x92150xc2x0 C. to +150xc2x0 C.
In accordance with the present invention, there is also provided a method of producing a block copolymer comprising the step of: in a hydrocarbon based solvent and using an organic active metal as an initiator,
(I) (i) first polymerizing a conjugated diene to produce a polymer block A of a conjugated diene, then (ii) polymerizing a mixture of a conjugated diene and an aromatic vinyl compound in the presence of the polymer block A having an active end or
(II) (i) first polymerizing a mixture of a conjugated diene and an aromatic vinyl compound to produce a random copolymer block B, and then (ii) polymerizing a conjugated diene in the presence of the copolymer block B having an active end to produce a polymer block A of a conjugated diene.
In accordance with the present invention, there is further provided a rubber composition comprising a block copolymer (1) containing a polymer block A of a conjugated diene and a random copolymer block B of a conjugated diene and an aromatic vinyl compound, (2) having a weight ratio (A:B) of the polymer block A and the copolymer block B of 5:95 to 95:5, (3) having a bound aromatic vinyl compound content in the copolymer block B of 1 to 50% by weight, (4) having a weight average molecular weight (Mw) of 100,000 to 5,000,000, and (5) having at least two transition points measured by differential scanning calorimetry in the range of xe2x88x92150xc2x0 C. to +150xc2x0 C. and at least one diene rubber.
In accordance with the present invention, there is further provided a method of producing a rubber composition comprising mixing a block copolymer (1) containing a polymer block A of a conjugated diene and a random copolymer block B of a conjugated diene and an aromatic vinyl compound, (2) having a weight ratio (A:B) of the polymer block A to the copolymer block B of 5:95 to 95:5, (3) having a bond aromatic vinyl content in the copolymer block B of 1 to 50% by weight, (4) having a weight average molecular weight (Mw) of 100,000 to 5,000,000, and (5) having at least two transition points measured by differential scanning calorimetry in the range of xe2x88x92150xc2x0 C. to +150xc2x0 C. and at least one diene rubber so as to produce a rubber composition, wherein at least one diene rubber (X) compatible with the polymer block A in the block copolymer and at least one diene rubber (Y) compatible with the copolymer block B are used as the diene rubber, the block copolymer is first mixed with either one compound of the diene rubber (X) component and diene rubber (Y) component of the diene rubber, followed by mixing with the other compound.
In accordance with the present invention, there is still further provided a rubber composition for a tire comprising (1) an incompatible polymer blend composed of at least two rubbers selected from natural rubber (NR), synthetic isoprene rubber (IR), polybutadiene rubber (BR), and styrene-butadiene copolymer rubber (SBR), wherein two polymer phases Xxe2x80x2 and Yxe2x80x2 are formed, and (2) 0.1 to 20 parts by weight, based on 100 parts by weight of the total rubber components including the block copolymer, of a block copolymer composed of a monomer selected from the group consisting of isoprene, butadiene, and styrene having at least two blocks Axe2x80x2 and Bxe2x80x2, the blocks Axe2x80x2 and Bxe2x80x2 being mutually incompatible, the block Axe2x80x2 being compatible with the polymer phase Xxe2x80x2 and incompatible with the polymer phase Yxe2x80x2, the block Bxe2x80x2 being compatible with the polymer phase Yxe2x80x2 and incompatible with the polymer phase Xxe2x80x2, wherein an amount of 1,4-bonds contained in the block Axe2x80x2 and Bxe2x80x2 is at least 50,000, when converted to the weight average molecular weight, and a ratio Axe2x80x2/Bxe2x80x2 of the amount of 1,4-bonds contained in the blocks Axe2x80x2 and Bxe2x80x2 is 0.67 to 1.50.
In accordance with the present invention, there is still further provided a pneumatic tire having a tread made of a rubber composition comprising 100 parts by weight of a rubber component composed of (a) 50 to 90 parts by weight of natural rubber and/or polyisoprene rubber, (b) 8 to 40 parts by weight of styrene-butadiene copolymer or polybutadiene rubber having a glass transition temperature of not more than xe2x88x9275xc2x0 C., and (c) 0.5 to 20 parts by weight of an Axe2x80x3-Bxe2x80x3 (or Bxe2x80x2xe2x80x3) type block copolymer composed of a block Axe2x80x3 of polyisoprene having a cis content of at least 70% by weight and a block Bxe2x80x3 of poly(styrene-butadiene) having a styrene content of less than 20% by weight and a 1,2-vinyl bond content of less than 50% or a block Bxe2x80x2xe2x80x3 of polybutadiene in which 35 to 55 parts by weight of carbon black having a CTAB surface area of more than 125 m2/g and a C-DBP oil absorption of 100 to 150 ml/100 g is formulated.
The present inventors engaged in intensive studies for achieving the above objects and, as a result, found that a novel block copolymer containing a polymer of a conjugated diene and a copolymer block of a conjugated diene and an aromatic vinyl compound and having at least two transition points measured by differential scanning calorimetry within the range of xe2x88x92150xc2x0 C. to +150xc2x0 C. exhibits a remarkable effect of improvement of the compatibility among different types of diene rubbers. We further found that a rubber composition composed of this block copolymer blended in a diene rubber has excellent vulcanization rate, improved property of the abrasion resistance or tensile strength, and further remarkably improved abrasion resistance and chipping resistance when formed into a pneumatic tire. Further, the abrasion resistance may be further improved by narrowing the distribution of the molecular weight (Mw/Mn) of the block copolymer. By fine tuning the method of production of the block copolymer or the method of preparation of the rubber composition, it is possible to further bring out the above-mentioned characteristic points.
The present invention was completed based on the above findings.
The block copolymer according to the first aspect of the present invention includes a polymer block A composed of a conjugated diene and a random copolymer block B of a conjugated diene and an aromatic vinyl compound and has at least two transition points measured by differential scanning calorimetry (DSC) within the range of xe2x88x92150xc2x0 C. to +150xc2x0 C. The number of transition points is preferably two. If there is only one transition point measured by DSC, it is not possible to sufficiently bring out the effect of compatibilization of insufficiently compatible or incompatible diene rubbers.
The conjugated diene used in the production of the block copolymer is not particularly limited. For example, 1,3-butadiene, 2-methyl-1,3-butadiene (that is, isoprene), 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene, 1,3-pentadiene, etc. may be mentioned. Among these, 1,3-butadiene, 2-methyl-1,3-butadiene, etc. are preferable. These conjugated dienes may be used alone or in any combinations of two or more.
The aromatic vinyl compound used in the production of the block copolymer is not particularly limited. For example, styrene, xcex1-methylstyrene, 2-methylstyrene, 3-methylstyrene, 4-methylstyrene, 2,4-diisopropylstyrene, 2,4-dimethylstyrene, 4-t-butylstyrene, 5-t-butyl-2-methylstyrene, monochlorostyrene, dichlorostyrene, monofluorostyrene, etc. may be mentioned. Among these, styrene is preferable. These aromatic vinyl compounds may be used alone or in any combinations of two or more.
The polymer block A is a polymer block of substantially only a conjugated diene. In particular, when a polymer block of 2-methyl-1,3-butadiene, is used, the vulcanization rate, abrasion resistance, and tensile strength are preferably balanced to a high degree. If an olefinic monomer such as an aromatic vinyl compound is bonded in a significant amount with the polymer block A, when the block copolymer obtained is used as a compatibilizing agent for two or more types of diene rubbers, the vulcanization rate becomes slower and the effect of improvement of the abrasion resistance, tensile strength, etc. can no longer be obtained.
The microstructure of the polymer block A portion is not particularly limited. For example, the bound vinyl content (i.e., the amount of 1,2-vinyl bonds+the amount of 3,4-vinyl bonds) is normally in the range of 1 to 80 mol %. The amount of vinyl bonds is preferably 2 to 50 mol %, more preferably 3 to 30 mol %. At this time, the tendency is that the vulcanization rate becomes better and, further, the abrasion resistance, tensile strength, etc. are improved to a high degree. The bound vinyl content is particularly preferably often 3 to 15 mol %. The remaining bonds other than the vinyl bonds are 1,4-bonds. The ratio of the cis-1,4-bonds and trans-1,4-bonds among them is suitably selected depending upon the purpose of use.
The amount of the bonded aromatic vinyl (S) in the copolymer block B is suitably selected in accordance with the purpose of use, but normally is 1 to 50% by weight, preferably 5 to 45% by weight, more preferably 20 to 40% by weight. When the bound aromatic vinyl content is in these ranges, the tensile strength of the rubber composition preferably becomes excellent.
The microstructure of the conjugated diene portion in the copolymer block B is suitably selected depending upon the purpose of use and is not particularly limited. For example, the bound vinyl content (V) (i.e., 1,2-vinyl bonds or 1,2-vinyl bonds+3,4-vinyl bonds) is normally 1 to 55 mol % of the bound conjugated diene units in the copolymer block B. The bound vinyl content is preferably 5 to 55 mol %, more preferably 10 to 50 mol %. When the bound vinyl content is within this range, the vulcanized properties and abrasion resistance of the rubber composition are improved. The bonds other than the vinyl bonds of the conjugated diene portion are 1,4-bonds. The ratio of the cis-1,4-bonds and trans-1,4-bonds in these bonds is suitably selected depending upon the purpose of use.
When the amount S and amount V in the copolymer block B are within the above range and the relation:
V less than 2S+17
stands, the effect of compatibilization of the incompatible diene rubbers is high. In particular, the abrasion resistance and tensile strength can be preferably improved to a high degree.
Further, the ratio between the amount of 1,4-bonds (A-1,4). in the polymer block A and the amount of 1,4-bonds (B-1,4) in the conjugated diene portion in the copolymer B is not particularly limited and is suitably selected depending upon the purpose of use, but the molar ratio of (A-1,4):(B-1,4) is normally 1:9 to 9:1, preferably 3:7 to 7:3, more preferably 4:6 to 6:4. When the ratio of the two is within this range, preferably the vulcanization rate is better and the effect of improvement of the tensile strength, abrasion resistance, etc. is also high.
The distribution of chains of the aromatic vinyl units in the copolymer block B is suitably selected depending upon the purpose of use, but independent chains with one bound aromatic vinyl measured by the ozone degradation method (hereinafter referred to as S1 chains) normally comprise at least 40% by weight of the total amount of bound aromatic vinyl, preferably at least 50% by weight, more preferably at least 60% by weight. Further, long chains having 8 bound aromatic vinyl units measured by the ozone degradation method (hereinafter referred to as S8 chains) normally comprise not more than 10% by weight of the total amount of bound aromatic vinyl, preferably not more than 5% by weight, more preferably 3% by weight. When the S1 chains and the S8 chains are within this range, preferably the heat buildup property is particularly excellent.
The weight ratio (A:B) of the polymer block A and the copolymer block B is normally 5:95 to 95:5, preferably 10:90 to 70:30, more preferably 20:80 to 50:50. When the ratio of the polymer block A and the copolymer block B is within this range, the vulcanization rate is preferably sufficiently improved.
The molecular weight of the block copolymer of the present invention is, in terms of weight average molecular weight (MW) measured by the gel permeation chromatography (GPC) method and converted to a polystyrene value, 100,000 to 5,000,000, preferably 300,000 to 3,000,000, more preferably 500,000 to 1,500,000. If the weight average molecular weight (Mw) is excessively small, it is difficult to sufficiently bring out the effect of compatibilization and the effect of improvement of the abrasion resistance, tensile strength, etc. is also poor, while conversely if excessively large, the processability is decreased. Accordingly, neither of these is preferred.
The distribution of the molecular weight of the block copolymer of the present invention is not particularly limited, but when the ratio (Mw/Mn) between the weight average molecular weight (Mw) and the number average molecular weight (Mn) measured by the above GPC method is normally not more than 2.5, preferably not more than 2.0, more preferably not more than 1.5, the effect of compatibilization is preferably high.
The method of production of the block copolymer of the present invention is not particularly limited, but for example it may be produced by the method of (1) polymerizing a conjugated diene to produce a polymer block A of the conjugated diene in a hydrocarbon solvent using an organic active metal as an initiator, then (2) polymerizing a mixture of a conjugated diene and aromatic vinyl compound in the presence of the polymer block A having an active end to produce a random copolymer block B (method a) or the method of (1) polymerizing a mixture of a conjugated diene and aromatic vinyl compound in a hydrocarbon solvent using an organic active metal as an initiator to produce a random copolymer block B, then (2) polymerizing a conjugated diene in the presence of the copolymer block B having an active end to produce a polymer block A of the conjugated diene (method b). The method a is particularly preferable from the viewpoint of the easy polymerization operation and the physical properties of the block copolymer obtained.
In particular, a block copolymer having a narrow molecular weight (Mw/Mn) distribution can be obtained by the method of charging a hydrocarbon solvent, Lewis base, and conjugated diene into the reaction system, then adding the organic active metal to initiate the polymerization reaction, producing the polymer A of the conjugated diene, then adding a mixture of a conjugated diene and aromatic vinyl for further polymerization etc.
As the organic active metal, for example, an anionically polymerizable organic active metal compound such as an organic alkali metal compound, an organic alkali earth metal compound, or an organic lanthanoid rare earth metal compound may be mentioned. Among these, an organic alkali metal compound is particularly preferred from the viewpoints of the polymerization reactivity, economy, etc.
As the organic alkali metal compound, for example, mono organolithium compounds such as n-butyl lithium, sec-butyl lithium, t-butyl lithium, hexyl lithium, phenyl lithium, and stilbene lithium; polyfunctional organolithium compounds such as dilithiomethane, 1,4-dilithiobutane, 1,4-dilithio-2-ethylcyclohexane, and 1,3,5-trilithiobenzene; sodium naphthalene, potassium naphthalene, etc. may be mentioned. Among these, organolithium compounds are preferred. Mono organolithium compounds are particularly preferred.
As the organic alkali earth metal compound, for example, n-butyl magnesium bromide, n-hexyl magnesium bromide, ethoxy calcium, calcium stearate, t-butoxy strontium, ethoxy barium, isopropoxy barium, ethyl mercapto barium, t-butoxy barium, phenoxy barium, diethyl amino barium, barium stearate, ethyl barium, etc. may be mentioned.
As the organic lanthanoid rare earth metal compound, for example, a composite catalyst composed of neodium basatate/triethyl aluminum hydride/ethyl aluminum sesquiochloride as described in Japanese Examined Patent Publication (Kokoku) No. 63-64444 etc. may be mentioned.
These organic active metals may be used alone or in any combinations of two or more. The amount of the organic active metal used is suitably selected depending upon the molecular weight of the produced polymer required and is normally in the range of 0.01 to 20 mmol per 100 g of total monomer, preferably 0.05 to 15 mmol, more preferably 0.1 to 10 mmol.
As the Lewis base, for example, an ether such as tetrahydrofuran, diethylether, dioxane, ethylene glycol dimethyl ether, ethylene glycol dibutyl ether, diethylene glycol dimethyl ether, and diethylene glycol dibutyl ether; a tertiary amine compound such as tetramethyl ethylene diamine, trimethyl amine, triethylene amine, pyridine, and kinucridine; an alkali metal alkoxide compound such as potassium-t-amyloxide, and potassium-t-butyloxide; a phosphine compound such as triphenyl phosphine; and other compounds may be mentioned. Among these, an ether or tertiary amine compound etc. are preferable.
These Lewis bases may be used alone or in any combinations of two or more. The amount of the Lewis base used is normally 0 to 200 moles per mole of the organic active metal, preferably 0.01 to 100 moles, more preferably 0.1 to 50 moles, most preferably 0.3 to 20 moles.
The polymerization reaction may be any of an isothermic reaction or adiabatic reaction and normally is performed in the range of polymerization temperature of 0xc2x0 C. to 150xc2x0 C., preferably 20xc2x0 C. to 120xc2x0 C. After the end of the polymerization reaction, an ordinary method is used, for example, an alcohol such as methanol or isopropanol is added as a terminator, to stop the polymerization reaction, an antioxidant (stabilizer) or cram agent is added, then the solvent is removed by direct drying, steam stripping, or another method and the produced polymer is recovered.
The rubber composition according to the first aspect of the present invention contains at least one diene rubber and the above block copolymer as essential ingredients.
As the diene rubber, to bring out the compatibilizing action of the block copolymer of the present invention, preferably a combination of at least two diene rubbers which are mutually incompatible or poorly compatible is used. More specifically, as the diene rubber, normally at least one diene rubber (X) compatible with the polymer block A in the block copolymer and at least one diene rubber (Y) compatible with the copolymer block B are included. As combinations of these diene rubbers (X) and (Y), for example, combinations of various types of rubbers used as rubber ingredients for tire treads may be mentioned.
Whether or not a diene rubber is compatible with the polymer block A or copolymer block B in the block copolymer of the present invention may be determined by DSC analysis of the blend. That is,
(1) The transition points of the block copolymer are measured by DSC to find the transition point (TA) based on the polymer block A and the transition point (TB) based on the copolymer block B.
(2) The transition point (TC) of the diene rubber C is found by DSC.
(3) The block copolymer and the diene rubber C are mixed (normally at 40 to 100xc2x0 C. for 5 to 15 minutes) and the mixture measured by DSC, wherein:
[1] When three transition points (TA, TB, and TC) are measured, the diene rubber C is determined to be incompatible with both the polymer block A and the copolymer block B.
[2] When the transition points of TA and TC disappear and a new transition point T occurs between TA and TC, the diene rubber C can be determined to be a diene rubber (X) compatible with the polymer block A.
[3] When the transition points of TB and TC disappear and a new transition point T occurs between TB and TC, the diene rubber C can be determined to be a diene rubber (Y) compatible with the copolymer block B.
More specifically, based on the above-mentioned determination method, as a preferable example of the diene rubber (X) compatible with the polymer block A in the block copolymer of the present invention, for example, natural rubber, polyisoprene rubber, isoprene-butadiene rubber, etc. may be mentioned. Among these, natural rubber, polyisoprene rubber having at least 70 mol % of 1,4-bonds, and butadiene-isoprene rubber having an isoprene content of at least 50% by weight and at least 70 mol % of 1,4-bonds are preferable. These diene rubbers (X) may be used alone or in any combinations of two or more.
As preferable specific examples of the diene rubber (Y) compatible with the copolymer block (B) in the block copolymer of the present invention, for example, styrene-butadiene rubber, styrene-isoprene rubber, styrene-isoprene-butadiene rubber, butadiene rubber, lower acrylonitrile-butadiene rubber, etc. may be mentioned. Among these, a styrene-butadiene rubber or styrene-isoprene-butadiene rubber having a styrene content of not more than 35% by weight and a bound vinyl content of the conjugated diene portion (i.e., the amount of 1,2-bonds or 1,2-bonds +3,4-bonds) of 17 to 70 mol % etc. are preferable and styrene-butadiene rubber having a bound styrene content of 10 to 30% by weight and a bound vinyl content of the conjugated diene portion of 30 to 65 mol % or a styrene-isoprene-butadiene rubber having a bound isoprene content of 1 to 10% by weight is more preferable.
These diene rubbers may be used alone or in any combinations of two or more. The ratio of the diene rubber (X) and the diene rubber (Y) is suitably selected depending upon the purpose of use and is, in terms of the weight ratio of X:Y, normally 5:95 to 95:5, preferably 10:90 to 90:10, more preferably 30:70 to 70:30.
The Mooney viscosity (ML1+4, 100xc2x0 C.) of the diene rubbers is suitably selected depending upon the purpose of use, but normally is 10 to 250, preferably 30 to 200, more preferably 40 to 150.
The ratio of the block copolymer formulated in the diene rubber component is suitably selected depending upon the purpose of use, but is normally 0.1 to 40 parts by weight based upon 100 parts by weight of the diene rubber component (i.e., total amount), preferably 1 to 30 parts by weight, more preferably 5 to 25 parts by weight. When the ratio of formulation of the block copolymer is within this range, the compatibilizing effect and vulcanized properties are preferably excellent.
The rubber composition of the present invention normally has a reinforcing agent blended therein. As the reinforcing agent, for example, carbon black, silica, etc. may be mentioned. The carbon black is not particularly limited, but for example Furnace Black, Acetylene Black, Thermal Black, Channel Black, graphite, etc. may be used. Among these, Furnace Black is particularly preferred. As specific examples, SAF, ISAF, ISAF-HS, ISAF-LS, IISAF-HS, HAF, HAF-HS, HAF-LS, FEF, and other various grades may be mentioned. These carbon blacks may be used alone or in any combinations of two or more. The nitrogen specific area (N2SA) of the carbon black is not particularly limited, but is normally 5 to 200 m2/g, preferably 50 to 150 m2/g, more preferably 80 to 130 m2/g. Further, the DBP oil absorption of the carbon black is not particularly limited, but is normally 5 to 300 ml/100 g, preferably 50 to 200 ml/100 g, more preferably 80 to 160 ml/100 g.
By using as the carbon black High Structure Carbon Black having a cetyl trimethyl ammonium bromide (CTAB) specific area of 110 to 170 m2/g and a DBP (24M4DBP) oil absorption after compression 4 times repeatedly at a pressure of 24,000 psi of 110 to 130 ml/100 g as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-230290, the abrasion resistance can be improved.
The silica is not particularly limited, but for example a dry type white carbon, wet type white carbon, colloidal silica, precipitated silica disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-62838, etc. may be mentioned. Among these, the wet type white carbon containing hydrous silica as its main ingredient is particularly preferable. The specific surface area of the silica is not particularly limited, but when specific surface area by nitrogen absorption (N2SA) (BET method) is normally 50 to 400 m2/g, preferably 100 to 250 m2/g, more preferably 120 to 190 m2/g, a sufficient improvement of the heat buildup property, tensile strength, processability, etc. is preferably achieved. Here, the specific surface area by nitrogen absorption is the value measured by the BET method based on ASTM D3037-81.
These reinforcing agents may be used alone or in any combinations of two or more. The ratio of formulation of the reinforcing agent is suitably selected depending upon the purpose of use, but normally is 10 to 200 parts by weight based on 100 parts by weight of the diene rubber component (total amount), preferably 20 to 150 parts by weight, more preferably 30 to 120 parts by weight.
The rubber composition of the present invention may optionally contain other compounding agents generally used in the rubber industry. As these other compounding agents, for example, a vulcanization agent, vulcanization accelerator, anti-aging agent, activator, plasticizer, lubricant, filler, etc. may be mentioned. The amount of formulation of these compounding agents is suitably selected in a range not adversely affecting the effects of the present invention.
The rubber composition of the present invention may be obtained by mixing (or kneading) the above ingredients according to an ordinary method. For example, at least one diene rubber, the block copolymer, and the compounding agents other than vulcanization agent and vulcanization accelerator may be mixed, then the vulcanization agent and vulcanization accelerator mixed into this mixture to obtain the rubber composition.
In particular, when the diene rubber to be mixed with the block copolymer is composed of a diene rubber (X) compatible with the polymer block A in the block copolymer and a diene rubber (Y) compatible with the copolymer block B, if either one component of the diene rubbers (X) and (Y) is mixed with the block copolymer, then the remaining diene rubber is mixed, the properties such as the tensile strength and the abrasion resistance can be further preferably improved.
The temperature of mixing of the diene rubber, block copolymer, and compounding agents other than the vulcanization agent and the vulcanization accelerator is normally room temperature to 250xc2x0 C., preferably 40xc2x0 C. to 200xc2x0 C., more preferably 50xc2x0 C. to 180xc2x0 C. The kneading time is normally at least 30 seconds, preferably 1 to 30 minutes. The vulcanization agent and the vulcanization accelerator are normally mixed after cooling to not more than 100xc2x0 C., preferably room temperature to 80xc2x0 C. The mixture is then press vulcanized at a temperature of normally 120xc2x0 C. to 200xc2x0 C., preferably 140xc2x0 C. to 180xc2x0 C.
The rubber composition for tire use according to the second aspect of the present invention is composed of formulating (1) an incompatible polymer blend of two polymer phases Xxe2x80x2 and Yxe2x80x2 (the preferable Xxe2x80x2/Yxe2x80x2 weight ratio being 5/95 to 95/5, more preferably 10/90 to 90/10) comprising at least two incompatible rubbers selected from NR, IR, BR, and SBR with (2) 0.1 to 20 parts by weight, preferably 0.3 to 18 parts by weight, based upon 100 parts by weight of the total rubber component including the block copolymer, of a block copolymer having at least two blocks composed of monomers selected from isoprene, butadiene, and styrene, wherein the blocks Axe2x80x2 and Bxe2x80x2 is mutually incompatible, the block Axe2x80x2 is compatible with the polymer phase Xxe2x80x2 and incompatible with the polymer phase Yxe2x80x2, the block Bxe2x80x2 is compatible with the polymer phase Yxe2x80x2 and incompatible with the polymer phase Xxe2x80x2, and the amount of 1,4-bonds contained in the blocks Axe2x80x2 and Bxe2x80x2 is at least 50,000 when converted to the weight average molecular weight, and the ratio Axe2x80x2/Bxe2x80x2 of the 1,4-bonds (converted to molecular weight) contained in the blocks Axe2x80x2 and Bxe2x80x2 is 0.67 to 1.50.
That is, the present inventors observed that addition of a suitable block copolymer when kneading a phase-separated polymer blend resulted in the block copolymer acting as a compatibilizing agent and increasing the fineness of the phase structure and reinforcing the phase separated interface, but found that, if the cross-linking bonds between the blocks of the block copolymer and the polymer forming the matrix were not effectively caused at the time of vulcanization, the deformation at the time of vulcanization caused the finer phase structure and reinforced interface to return to their original states and the effect of improvement of the breaking strength etc. is decreased. They engaged in in-depth studies on prevention of this decrease in the effect of improvement of the breaking strength and, as a result, found that it was possible to solve the abovementioned problems by specifying the amounts and ratio of the 1,4-bonds contained in the blocks of the block polymer and causing the cross-linking reaction between the blocks and the matrices to proceed at substantially the equivalent rates.
The blocks Axe2x80x2 and Bxe2x80x2 of the block copolymers used in the present invention have to be mutually incompatible or else the block copolymer molecules cannot sufficiently invade the matrix phases Xxe2x80x2 and Yxe2x80x2, and therefore, the desired effect of improvement of the breaking properties cannot be obtained. Further, in the present invention, the block Axe2x80x2 must be compatible with the polymer phase Xxe2x80x2 and incompatible with the polymer phase Yxe2x80x2, while the block Bxe2x80x2 must be compatible with the polymer phase Yxe2x80x2 and incompatible with the polymer phase Xxe2x80x2. When this relationship is not maintained, the block copolymer cannot be positioned at the Xxe2x80x2 and Yxe2x80x2 phase separated interface, and therefore, the phase separated interface is not reinforced and a sufficient effect of improvement of the breaking strength cannot be unpreferably obtained.
The amount of the 1,4-bonds contained in the blocks Axe2x80x2 and Bxe2x80x2 of the block copolymer used in the present invention has to be, when converted into the weight average molecular weight, at least 50,000, preferably at least 55,000, or else a sufficient cross-linkability with the polymer component forming the matrix cannot be obtained. Therefore, in the same way as the above case, a sufficient reinforcement of the phase separated interface is not obtained and the desired effect of improvement of the breaking strength is not unpreferably obtained.
The ratio Axe2x80x2/Bxe2x80x2 of the amounts of 1,4-bonds (when converted to molecular weight) contained in the blocks Axe2x80x2 and Bxe2x80x2 of the block copolymer used in the present invention is 0.67 to 1.50, preferably 0.70 to 1.40. If the ratio Axe2x80x2/Bxe2x80x2 is out of this range, a difference in the progress of the cross-linking arises between the blocks Axe2x80x2 and Bxe2x80x2 of the block copolymer and the matrix phases Xxe2x80x2 and Yxe2x80x2 at the time of the vulcanization reaction, the phase separated structure made finer in the kneading process reagglomerates and enlarges, and therefore, the desired effect of improvement of the breaking strength is not unpreferably obtained.
Further, showing the method of calculation of the amount of 1,4-bonds (when converted to molecular weight), if the styrene content (wt %) of a block measured in the process of polymerization of the block copolymer is St, the content of the vinyl polymerization unit of the conjugated diene polymer portion (total amount of 1,2-vinyl content (mol %) and 3,4-vinyl content (mol %) is Vn, the weight average molecular weight of the block copolymer as a whole is Mw, and the weight ratio of the block i is Wi, the amount of the 1,4-bonds of the block i (converted to molecular weight) is calculated by the following formula:
Amount of 1,4-bonds (when converted to molecular weight)=Mwxc3x97Wixc3x97((100-St)/100)xc3x97((100-Vn)/100)
The method of production of the block copolymer (i) of the present invention is not particularly limited, but for example it may be produced by the method of polymerization of a monomer such as isoprene, butadiene, and styrene in a hydrocarbon solvent using an organic active metal as an initiator.
As the organic active metal, for example, an anionically polymerizable organic active metal such as an organic alkali metal compound, organic alkali earth metal compound, and organic lanthanoid rare earth metal compound may be mentioned. Among these, an organic alkali metal compound is particularly preferable.
According to the present invention, the block copolymer is formulated in an amount of 0.1 to 20 parts by weight, preferably 0.3 to 18 parts by weight, based upon 100 parts by weight of the total amount of the incompatible polymer blend and block copolymer. If the amount of formulation of the block copolymer is more than 20 parts by weight, the viscoelasticity of the block copolymer is adversely affected, and therefore, the originally intended balance of the wet braking property and tumbling resistance is unpreferably changed.
The incompatible polymer blend composed of the polymer phases Xxe2x80x2 and Yxe2x80x2 used in the present invention is not particularly limited so long as at least two types of polymers selected from NR, IR, BR, and SBR are selected to form two incompatible polymer phases Xxe2x80x2 and Yxe2x80x2. Further, the block copolymer composed of the blocks Axe2x80x2 and Bxe2x80x2 used in the present invention may be made any polymer provided with the above conditions. For example, a BR block, SBR block, IR block, SIR (styrene-isoprene rubber) block, BIR (butadiene-isoprene) block, SBIR (styrene-butadiene-isoprene) block, etc. may be suitably combined for use.
Specific examples of combinations of the incompatible polymers and block copolymer are given below:
A composition wherein the polymer phase Xxe2x80x2 is composed of polybutadiene (BR) having a cis content of at least 80% by weight, preferably 85 to 100% by weight, the polymer phase Yxe2x80x2 is composed of natural rubber (NR) and/or a synthetic isoprene rubber (IR), and the blocks Axe2x80x2 and Bxe2x80x2 of the block copolymer are SBR or BR having the following composition:
Axe2x80x2: St=0 to 35% by weight (preferably 5 to 35% by weight), Vn=5 to 80 mol % (preferably 8 to 80 mol %), and Vnxe2x89xa62St+30
Bxe2x80x2: St=0 to 30% by weight (preferably 5 to 30% by weight), Vn greater than 2St+30
wherein St indicates the styrene content and Vn indicates the vinyl content of the butadiene part.
A rubber composition wherein the polymer phase Xxe2x80x2 is composed of styrene-butadiene rubber (SBR) and/or polybutadiene rubber (BR), the polymer phase Yxe2x80x2 is natural rubber (NR) and/or synthetic isoprene rubber (IR), the block Axe2x80x2 of the block copolymer is SBR or BR having the following composition, and the block Bxe2x80x2 is polyisoprene (IR) having the following composition:
Axe2x80x2: (St)=0 to 50% by weight (preferably 5 to 50% by weight), Vn=5 to 70 mol % (preferably 8 to 70 mol %), and Vnxe2x89xa62St+30
Bxe2x80x2: 1,4-bondsxe2x89xa770% by weight (preferably 72 to 100% by weight)
wherein St indicates the styrene content and Vn indicates the vinyl content.
A composition wherein the polymer phase Xxe2x80x2 is composed of polybutadiene (BR) having at least 80% by weight cis content (preferably 85 to 100% by weight), the polymer phase Yxe2x80x2 is composed of natural rubber (NR) and/or synthetic isoprene rubber (IR), the block Axe2x80x2 of the block copolymer is composed of SBR or BR having the following composition, and the block Bxe2x80x2 is a polyisoprene (IR) having the following composition:
Axe2x80x2: St=0 to 35% by weight (preferably 5 to 35% by weight), Vn=5 to 80 mol % (preferably 8 to 80 mol %), and Vnxe2x89xa62St+30
Bxe2x80x2: 1,4 bondsxe2x89xa770% by weight (preferably 72 to 100% by weight)
wherein St indicates the styrene content and Vn indicates the vinyl content.
A composition wherein the polymer phase Xxe2x80x2 is composed of a polybutadiene (BR) having at least 80% by weight cis content (preferably 85 to 100% by weight), the polymer phase Yxe2x80x2 is composed of a styrene content of 5 to 60% by weight, a vinyl content of 5 to 35 mol % or 65 to 85 mol % or a styrene-butadiene rubber (SBR) having a styrene content of 35 to 60% by weight and a vinyl content of 35 to 65 mol %, the block Axe2x80x2 of the block copolymer is a styrene-butadiene rubber (SBR) or polybutadiene rubber (BR) having the following composition, and the block Bxe2x80x2 is a styrene-butadiene rubber (SBR) having the following composition:
Axe2x80x2: St=0 to 35% by weight, Vn=5 to 80 mol %
Bxe2x80x2: St=5 to 60% by weight, Vn=5 to 35 mol % or 65 to 85 mol % or St=35 to 60% by weight, Vn=35 to 65 mol %
wherein St indicates the styrene content and Vn indicates the vinyl content of the butadiene part.
The rubber composition for a tire according to the second aspect of the present invention contains it at least 30 parts by weight, preferably 40 to 150 parts by weight, of a reinforcing filler in general use in the related art such as carbon black and/or silica. The carbon black and silica used may both be any one of those generally formulated into rubber compositions in the past.
Further, a softening agent, anti-aging agent, vulcanization adjuvant, wax, resin, and vulcanization compounding agent normally used in the rubber industry may be suitably used. Further, a foaming agent, low moisture plasticizer, staple fibers, etc. used in studless tires etc. in the past may also be used.
In blending the rubber composition for a tire according to the present invention, first, preferably, the rubber (i.e., matrix rubber and block copolymer) and the reinforcing filler and other compounding agents, other than the vulcanizing compounding agents, are mixed by an ordinary method, then the vulcanizing compounding agents are blended. Of course, it goes without saying that separate formulation of part of these ingredients falls in the technical scope of the present invention so long as the object of the present invention is not impaired. Further, the blending means may be one of the prior art.
The formulation of the rubber composition for a tire of the present invention may be vulcanized by a general method. The amount of formulation of the additives mentioned above may also be in general amounts. For example, it is preferable that the amount of formulation of sulfur is at least 0.5 part by weight, based on 100 parts by weight of the rubber component, more preferably 1.0 to 5.0 parts by weight. The vulcanization conditions of the rubber composition for a tire of the present invention may also use the general conditions.
The pneumatic tire according to a third aspect of the present invention is, as explained above, made of a rubber composition comprised of the mutually incompatible (i) NR and/or IR and (ii) SBR or BR in which is formulated the specific Xxe2x80x3-Yxe2x80x3 (or Yxe2x80x2xe2x80x3) type block copolymer compatible with the same and in which a specific fine carbon black is formulated, and therefore, the phase structure of these rubber ingredients becomes finer (ideally, they become compatible), a uniform rubber phase is formed, and the carbon black is uniformly dispersed in the phase. By using this rubber composition to make the tread of the tire, it is possible to enhance the breaking strength and breaking energy and possible to improve the abrasion resistance and the chipping resistance.
The NR, IR, SBR having the glass transition temperature (Tg) of not more than xe2x88x9275xc2x0 C., and the BR having the glass transition temperature (Tg) of not more than xe2x88x9275xc2x0 C. used in the third aspect of the present invention may be generally commercially available products. Here, the Tg is made not more than xe2x88x9275xc2x0 C. because if more than xe2x88x9275xc2x0 C., the abrasion resistance and the heat buildup property declinexe2x80x94neither of which is preferable.
The Xxe2x80x3-Yxe2x80x3 (or Yxe2x80x2xe2x80x3) type block copolymer is a block copolymer composed of the following block Xxe2x80x3 and block Yxe2x80x3 or block Yxe2x80x2xe2x80x3.
Block Xxe2x80x3
A block of IR having a cis content of at least a 70% by weight. If the cis content is less than 70% by weight, the compatibility with NR or IR becomes poor, and therefore, the anticipated increased fineness of the phase structure can no longer be obtained.
Blocks Yxe2x80x3 and Yxe2x80x2xe2x80x3
A block of a poly(styrene-butadiene) having a styrene content of less than 20% by weight and a bound 1,2-vinyl content of less than 50%, or a polybutadiene. With a styrene content of 20% by weight or more, the polymer becomes incompatible with SBR or BR having a Tg of not more than xe2x88x9275xc2x0 C., and therefore the intended effect cannot be obtained. Further, with a bound 1,2-vinyl content of 50% or more, the polymer becomes incompatible with SBR or BR having a Tg of not more than xe2x88x9275xc2x0 C., and therefore, the anticipated effect cannot be unpreferably obtained.
The ratio (weight ratio) of the Xxe2x80x3/Yxe2x80x3 (or Yxe2x80x2xe2x80x3) in the Xxe2x80x3-Yxe2x80x3 (or Yxe2x80x2xe2x80x3) type block copolymer should be 20 to 80/80 to 20. Outside of this range, the interlocking with the matrix rubber (NR, IR, SBR having a Tg of not more than xe2x88x9275xc2x0 C., and BR having a Tg of not more than xe2x88x9275xc2x0 C.) becomes smaller and the contribution to the compatibilization of the matrix rubber becomes insufficient. Further, the molecular weight of the Xxe2x80x3-Yxe2x80x3 (or Yxe2x80x2xe2x80x3) type block copolymer should be at least 30,000, preferably 50,000 to 800,000. If less than 30,000, the interlocking with the matrix becomes smaller and the co-crosslinkability is also unpreferably decreased.
The Xxe2x80x3-Yxe2x80x3 (or Yxe2x80x2xe2x80x3) type block copolymer is generally produced using an organic alkali metal compound catalyst such as butyl lithium in an organic solvent such as hexane by, for example, polymerizing isoprene to produce the block Xxe2x80x3 and further adding to this block in the terminal living state a styrene and butadiene or butadiene alone to produce the block Yxe2x80x3 or the block Yxe2x80x2xe2x80x3. At the time of this production, the ratio of formulation of the monomers, the vinylizing agent, polymerization conditions, etc. may be suitably selected as desired to obtain the desired block copolymer.
Further, Xxe2x80x3-Yxe2x80x3 (or Yxe2x80x2xe2x80x3) type block copolymers may be coupled by, for example, tin tetrachloride, silicon tetrachloride, etc. Further, as a separate method, the block Xxe2x80x3 and the block Yxe2x80x3 or the block Yxe2x80x2xe2x80x3 may be separately produced by ordinary methods, then coupled using a coupling agent such as tin tetrachloride or silicon tetrachloride.
The Xxe2x80x3-Yxe2x80x3 (or Yxe2x80x2xe2x80x3) type block copolymer may be terminal modified by a modifiying agent such as a compound having bonds shown by for example the following formula: 
wherein M indicates an O atom or S atom for example, an amide compound, an imide compound, a lanthanum compound, or a urea compound. The terminal modification may be performed after the end of the copolymerization of the Xxe2x80x3-Yxe2x80x3 (or Yxe2x80x2xe2x80x3) type block copolymer by the addition of a suitable modifying agent in the living state.
Further, the carbon black used in the third aspect of the present invention must be those having a CTAB surface area of more than 125 m2/g, preferably 125 to 170 m2/g, and a C-DBP oil absorption of 100 to 150 ml/100 g, preferably 110 to 130 mol/100 g. If a CTAB surface area is not more than 125 m2/g, the heat buildup property becomes lower, but unpreferably it is not possible to improve both of the properties of the abrasion resistance and the chipping resistance. If a C-DBP oil absorption is less than 100 ml/100 g, the improvement of the abrasion resistance is insufficient, while if more 150 ml/100 g, the elongation declines and there is a remarkable reduction in the chipping resistance. The CTAB surface area is measured, according to ASTM D 3765-80. The C-DBP oil absorption is also called the 24M4DBP oil absorption and is measured, according to ASTM D 3493.
In the present invention, a pneumatic tire is formed by using, as a tread, a rubber composition composed of 100 parts by weight of a rubber component composed of (i) 50 to 90 parts by weight of NR and/or IR, (ii) 8 to 40 parts by weight of SBR or BR having a Tg of not more than xe2x88x9275xc2x0 C., and (iii) 0.5 to 20 parts by weight of the above Xxe2x80x3-Yxe2x80x3 (or Yxe2x80x2xe2x80x3) type block copolymer in which 35 to 55 parts by weight of carbon black is formulated. If the ratio of formulation is outside of this range, there is insufficient increase in fineness of compatibility of the phase structure of the matrix rubber. Note that the block copolymer used in the present invention gives a sufficient effect even in small amounts. According to the present invention, it is possible to produce a pneumatic tire by the same method as in the past using this rubber composition as a tread.
The above rubber composition may, optionally contain, the compounding agents generally formulated into tire use and other rubber compositions such as sulfur, vulcanization accelerators, anti-aging agents, fillers, softening agents, and plasticizers.